Add tests

[melange396]

This repository is basically devoid of any kind of tests (automated or otherwise), except for the CI action that makes sure it builds. Unit tests would be nice, and something like integration tests would be fantastic... But at the very least, we should try to include a few [detailed] smoke tests to mimic the manual poking around that a developer or releaser is supposed to do. They could be automatically run against a PR's associated netlify preview. This can help if/when one of us "forgets" to do those manual checks, or neglects doing them thoroughly. There are many ways to approach this, from curl to selenium, but anything is hopefully better than the nothing that exists now.

Associated issues in other repos:

• Add tests www-main#1031
• Add tests www-covidcast#1261